<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<style>
  .slider-bar {
    position: absolute;
    left: 50%;
    top: 300px;
    margin-left: 600px;
    width: 45px;
    height: 130px;
    background-color: rgb(137, 48, 254);
  }

  .header {
    height: 150px;
    background-color: aqua;
  }

  .banner {
    height: 250px;
    background-color: pink;
  }

  .main {
    height: 1000px;
    background-color: bisque;
  }

  .w {
    width: 1200px;
    margin: 10px auto;
  }

  span {
    display: none;
    position: absolute;
    bottom: 0;
  }
</style>

<body>
  <div class="slider-bar">
    <span class="goBack">返回顶部</span>
  </div>
  <div class="header w">头部区域</div>
  <div class="banner w">banner区域</div>
  <div class="main w">主体部分</div>
  <script>
    //1.获取元素
    var sliderbar = document.querySelector('.slider-bar');
    var banner = document.querySelector('.banner');
    var sliderbarTop = sliderbar.offsetTop - banner.offsetTop;//当我们侧边栏固定定位之后应该变化的数值
    var main = document.querySelector('.main');
    var goBack = document.querySelector('.goBack');
    //2.页面滚动事件 scroll
    window.onscroll = function () {
      //console.log(window.pageYOffset);页面被卷去的部分
      //3.当我们页面被卷去的头部≥可被卷去的头部大小，侧边栏改为固定定位
      if (window.pageYOffset >= banner.offsetTop) {
        sliderbar.style.position = 'fixed';
        sliderbar.style.top = sliderbarTop + 'px';
      } else {
        sliderbar.style.position = 'absolute';
        sliderbar.style.top = '300px';
      }
      //4.当我们页面滚到main模块，就显示返回顶部goBack
      if (window.pageYOffset >= main.offsetTop) {
        goBack.style.display = 'block';
      } else {
        goBack.style.display = 'none';
      }
    }
  </script>
</body>

</html>